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<H3>fix drag command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID drag x y z fmag delta 
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>drag = style name of this fix command
<LI>x,y,z = coord to drag atoms towards
<LI>fmag = magnitude of force to apply to each atom (force units)
<LI>delta = cutoff distance inside of which force         is not applied (distance units) 
</UL>
<P><B>Examples:</B>
</P>
<PRE>fix center small-molecule drag 0.0 10.0 0.0 5.0 2.0 
</PRE>
<P><B>Description:</B>
</P>
<P>Apply a force to each atom in a group to drag it towards the point
(x,y,z).  The magnitude of the force is specified by fmag.  If an atom
is closer than a distance delta to the point, then the force is not
applied.
</P>
<P>Any of the x,y,z values can be specified as NULL which means do not
include that dimension in the distance calculation or force
application.
</P>
<P>This command can be used to steer one or more atoms to a new location
in the simulation.
</P>
<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
</P>
<P>No information about this fix is written to <A HREF = "restart.html">binary restart
files</A>.  None of the <A HREF = "fix_modify.html">fix_modify</A> options
are relevant to this fix.
</P>
<P>This fix computes a global 3-vector of forces, which can be accessed
by various <A HREF = "Section_howto.html#howto_15">output commands</A>.  This is the
total force on the group of atoms by the drag force.  The vector
values calculated by this fix are "extensive".
</P>
<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
the <A HREF = "run.html">run</A> command.  This fix is not invoked during <A HREF = "minimize.html">energy
minimization</A>.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B> 
</P>
<P><A HREF = "fix_spring.html">fix spring</A>, <A HREF = "fix_spring_self.html">fix spring/self</A>, 
<A HREF = "fix_spring_rg.html">fix spring/rg</A>, <A HREF = "fix_smd.html">fix smd</A>
</P>
<P><B>Default:</B> none
</P>
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